1. Technical Field
The present disclosure generally relates to formation evaluation and in particular to methods and apparatus for formation evaluation after drilling.
2. Background Information
Oil and gas wells have been drilled at depths ranging from a few thousand feet to as deep as 5 miles. Wireline and drilling tools often incorporate various sensors, instruments and control devices in order to carry out any number of downhole operations. These operations may include formation testing and monitoring and tool monitoring and control.
Typical well development includes a primary drilling system that drills the well through one or more producing formations to a total depth (“TD”). Formation evaluation tools used in a while-drilling arrangement provide some information relating to the traversed formations, but relatively slow transmission methods during drilling result limitations on the information available in real time. As a result, the knowledge gained from while-drilling tools provides at best a rough estimate as to the content and production ability of the formations. Furthermore, the primary drilling operation is extremely harsh on sensitive test instruments and protecting some of the more sensitive instruments from the shock and vibration environment experienced during primary drilling is expensive and sometimes limited by the space requirements in the drill string. Completion operations and the design of production processing facilities at the well head require information about the producing formations that is more precise and complete than provided by current while-drilling formation evaluation tools.
Wireline systems are sometimes used after primary drilling operations are complete at least through a suspected producing zone or zone of interest to gather more information about the zone of interest in order to better design the completion operations and surface processing facilities. The drill string is tripped and the wireline is run into the well to the zone of interest. The wireline tool provides a communication cable to the surface that provides information transmission rates higher than mud pulse telemetry and other while-drilling transmission methods. Wireline systems, however, cannot be conveyed into ultra-deep wells (up to 30,000 feet and more) without running the risk of losing the tool in the borehole due to strength limitations of the supporting cable. A broken wireline cable and lost tool may cost millions of dollars in lost time, lost equipment and the cost of drilling a bypass borehole.
Pipe-conveyed logging (“PCL”) tools are sometimes used to convey a formation evaluation tool in wells too deep for conventional wireline tools. A PCL tool includes a cable like a wireline tool, but the weight of the PCL tool is supported by a pipe allowing deeper penetration. Using a pipe also provides the ability to push the PCL tool in boreholes deviated from vertical. These PCL tools suffer in that the sensitive instruments can be damaged or destroyed if the operator pushes the pipe too hard through a well borehole zone that is of poor dimensional quality. Therefore, the PCL tool is typically lowered or pushed very slowly, which is a time cost. Even when the pipe is lowered slowly, an obstruction may still exist in the pre-drilled borehole. The operator must choose between tripping the PCL tool and increasing the weight on the pipe to force the pipe through the obstruction. There is a risk that the PCL tool becomes stuck in the borehole or even broken off due to the attempted forcing. This results in the need to drill a bypass borehole, which may cost the drilling operations millions of dollars in lost time and equipment.